1. Field of the Invention
The present invention relates to automotive automatic transmissions of an engaging pressure electronically controlled type (which will be referred to EPEC hereinafter for ease of description) wherein the hydraulic pressure needed by each frictionally engaging element is directly controlled by an electronic control means, and more particularly to a hydraulic control system of such type automotive automatic transmissions.
2. Description of the Prior Art
In order to clarify the task of the present invention, two prior art hydraulic control systems of an automotive automatic transmission will be briefly described with reference to attached drawings.
One is the hydraulic control system disclosed by Japanese Patent First Provisional Publication 10-159960, which employs two pressure control valves for changing the gain of hydraulic pressure applied to a clutch. As is seen from FIG. 9 of the accompanying drawings, the system comprises a main pressure control valve 100 which is actuated by a solenoid 102 and a sub-pressure control valve 104 which is actuated in accordance with an output pressure of the main control valve 100. Between output passages of these two control valves 100 and 104, there is arranged a ball valve 106. From the ball valve 106, there extends a passage to a frictionally engaging element, that is, clutch 108. The ball valve 106 is so arranged that when the output pressure of the main control valve 110 is higher than that of the sub-control valve 104, the output pressure of the main control valve 100 that has a smaller gain is applied to the clutch 108, while when the output pressure of the sub-control valve 104 is higher than that of the main control valve 100, the output pressure of the sub-control valve 104 that has a larger gain is applied to the clutch 108. With this arrangement, the system of the publication exhibits such a hydraulic pressure characteristic as shown in FIG. 10.
However, the system of the publication tends to have the following drawback due to its inherent construction. That is, as is indicated by the characteristic line part indicated by xe2x80x9c2-characteristicxe2x80x9d, in a second zone, that is, the zone between a point where the characteristic of the hydraulic pressure changes from xe2x80x9c1-characteristicxe2x80x9d to xe2x80x9c2-characteristicxe2x80x9d and a point where a maximum hydraulic pressure is needed, it is desirable to moderate the gain of the hydraulic pressure by a certain degree to obtain a satisfied clutch pressure irrespective of the unstable actuation by the solenoid 102. However, in practice, there is a need of reducing the size of the solenoid 102 by a certain degree due to inevitable constraint by cost and mounting space. As is known, reduction in size of the solenoid 102 brings about a constrain on the electric current zone of the solenoid 102. Accordingly, in order to obtain a maximum hydraulic pressure needed by the clutch at the constrained electric current zone, it is inevitably necessary to increase the gain of the hydraulic pressure of the clutch relative to the current applied to the solenoid 102. By this reason, at a gear change to a gear position that needs a larger torque contribution in the second zone, a satisfactorily fine pressure control to the clutch has not been obtained.
The other hydraulic control system is disclosed by Japanese Patent First Provisional Publication 10-103381, that is shown in FIG. 11 of the accompanying drawings. In this system, a switch valve 110 is arranged in the passage between the pressure control valve 100 and the clutch 108, and two clutch pistons 112 and 114 are associated with the clutch 108, as shown. With this arrangement, the system is so operated that in a lower gear position that needs a larger torque contribution, a higher torque capacity is obtained, while in a higher gear position that needs only a smaller torque contribution, a lower torque capacity is obtained. However, due to the nature of the arrangement wherein the engaging torque capacity of the clutch is changed by the two clutch pistons 112 and 114, that is, by using a so-called piston pressure receiving area switching mechanism, a fine control of the hydraulic pressure at the time of gear change has not been obtained. Furthermore, due to usage of the two clutch pistons 112 and 114, the entire length of an associated transmission becomes longer and the number of parts used is increased inevitably.
It is therefore an object of the present invention to provide a hydraulic control system of an automotive automatic transmission, which is free of the above-mentioned drawbacks.
That is, in accordance with the present invention, there is provided a hydraulic control system of an automotive automatic transmission, that can suitably set the torque capacity for the frictionally engaging element in accordance with a torque contribution needed by the engaging element when performing a given gear position and can finely control the hydraulic pressure applied to the engaging element.
According to a first aspect of the present invention, there is provided a hydraulic control system for use in an automatic transmission having a frictionally engaging element that is hydraulically actuated for assuming a plurality of gear positions. The hydraulic control system comprises a first hydraulic section that produces a line pressure; a solenoid valve that produces a solenoid pressure upon receiving a solenoid signal; a first pressure control valve that produces a first output pressure from the line pressure by using the solenoid pressure and the first output pressure as operation signal pressures, the first output pressure having a smaller gain with respect to the solenoid pressure; a second pressure control valve that produces a second output pressure from the line pressure by using the solenoid pressure and the second output pressure as operation signal pressures, the second output pressure having a larger gain with respect to the solenoid pressure; and a switching valve that selectively assumes a first position to feed the engaging element with the first output pressure when the engagement of the engaging element brings about a higher gear position that needs a smaller torque contribution, and a second position to feed the engaging element with the second output pressure when the engagement of the engaging element brings about a lower gear position that needs a larger torque contribution.
According to a second aspect of the present invention, there is provided a hydraulic control system for use in an automatic transmission having a low clutch that assumes its engaged condition at a plurality of lower gear positions of D-range and a high clutch that assumes its engaged condition at a plurality of higher gear positions of D-range. The hydraulic control system comprises a first hydraulic section that produces a line pressure; a solenoid valve that produces a solenoid pressure upon receiving a solenoid signal; a first pressure control valve that produces a first output pressure from the line pressure by using the solenoid pressure and the first output pressure as operation signal pressures, the first output pressure having a smaller gain with respect to the solenoid pressure; a second pressure control valve that produces a second output pressure from the line pressure by using the solenoid pressure and the second output pressure as operation signal pressures, the second output pressure having a larger gain with respect to the solenoid pressure; a switching valve including an axially movable spool, the spool selectively assuming a first position to feed the low clutch with the first output pressure when the engagement of the low clutch brings about a higher gear position that needs a smaller torque contribution, and a second position to feed the low clutch with the second output pressure when the engagement of the low clutch brings about a lower gear position that needs a larger torque contribution; and a second hydraulic section that comprises a first part that applies a predetermined fail pressure to one end of the spool, the predetermined fail pressure being the same as the maximum value of the high clutch pressure that is applied to said high clutch when a higher gear position of D-range is needed, a second part that applies said high clutch pressure to the other end of the spool, and a biasing spring that applies a biasing force to the other end of the spool in a manner to assist the second part.